1. Field of the Invention
The present invention relates to a defect detecting method for checking defect of an optical fiber, which is suitable for in-line inspection for inspecting an optical fiber during a drawing operation.
2. Description of the Related Art
If an optical fiber has internal defect such as cavity, it will result in undesired problems such as increase in light transmission loss and/or reduction of mechanical strength and/or poor end fusion between fibers. To cope with this, in optical fiber drawing equipment, such cavity defect has been detected in an in-line manner during a drawing operation.
For example, as disclosed in Japanese Patent Laid-open No. 4-106448 (1992) or as shown in FIG. 9A, there has been proposed a system in which, during the drawing operation, a raw optical fiber to be measured (referred to merely as xe2x80x9coptical fiber to be measuredxe2x80x9d hereinafter) A is illuminated by a laser beam B from a lateral direction, and forward scattering light C passed through the optical fiber A and scattered forwardly is received by an image sensor D such as a CCD line sensor, and a signal from the sensor is processed in a signal processing portion E and a judgement processing portion F, thereby obtaining an intensity distribution pattern G of the forward scattering light C and detecting defect of the optical fiber A to be measured on the basis of the pattern G.
In the signal processing portion E, light intensity of the scattered light C received by the image sensor D is read out along a scanning line direction (as shown by the arrow a in FIG. 9A), thereby outputting the light intensity distribution pattern G. In this case, a laser beam B straightly emitted from a light source (not shown) (scattering light C having zero displacement angle) is received by the image sensor D at its central portion in the scanning line direction, and the scattering light C scattered by the optical fiber to be measured is received at both sides of the central portion of the sensor, so that, in a graph as shown in FIG. 9B having the ordinate indicating a sensor position co-ordinate (displacement angle of the scattering light) and the abscissa indicating light intensity, the intensity distribution pattern becomes a mountain-like pattern G having the peak in the vicinity of zero (0) sensor position co-ordinate.
In the judgement processing portion F, the intensity distribution pattern G caught by the image sensor D is compared with an intensity distribution pattern (reference pattern) of a normal (correct) optical fiber previously measured, and accord or discord between both patterns are judged on the basis of a certain threshold value, thereby determining whether the optical fiber A to be measured is normal or abnormal. In this case, the reference pattern is obtained from a pattern of the forward scattering light obtained by illuminating light onto an optical fiber which was already determined to have no defect by an appropriate method. Although the reference pattern may be obtained by examining intensity distribution patterns of forward scattering lights regarding a plurality of optical fibers judged as normality and by averaging the results, in any case, a fixed reference pattern is used.
In the on-line measurement during the drawing operation, the optical fiber A to be measured may be vibrated or the position of the fiber may be shifted or the outer diameter of the fiber is changed or the intensity of the laser beam is changed, with the result that various changing factors are added to the intensity distribution pattern G to actually be processed in the judgement processing portion F. For example, as shown in FIG. 10A, the pattern G is totally moved upwardly or downwardly, or, as shown in FIG. 10B, the pattern is totally moved to left or right, or, as shown in FIG. 10C, inclination of the pattern G is changed (in these Figures, only positive part or negative part of the pattern G regarding the zero (0) sensor position co-ordinate is illustrated). As a result, when the fixed reference pattern is used, even if the optical fiber A to be measured itself has no abnormality, there may be great difference between the reference pattern and the pattern G obtained from the measurement, with the result that it may be judged as abnormality. To avoid this, generally, in consideration of such external changing factors, a certain width (play) is given to the reference pattern so that, so long as the measured pattern G is within a certain range (within threshold value) of the reference pattern, the optical fiber A to be measured is judged as normality.
However, recently, it has been ascertained that, in some cases, such play overlooks defect which should naturally be detected. For example, in the sectional position of the optical fiber, if there is the defect in the vicinity of the core, the difference between the measured pattern and the reference pattern becomes smaller in comparison with a case where the defect exists remote from the core, with the result that the difference may be included within the play of the threshold value, and, thus, the measured pattern may be judged as normality, which should naturally be judged as abnormality. Accordingly, there is need for providing algorism eliminating the changing factors as shown in FIGS. 10A, 10B and 10C not relating to the defect of the optical fiber, thereby permitting the correct detection of abnormality of the optical fiber A to be measured.
Due to recent software and/or hardware development regarding the digital signal processing ability including image processing, the reference level can be changed to cancel the changing factors not relating to the defect of the optical fiber. However, since further increase in drawing speed in order to reduce the manufacturing cost of the optical fiber has still been requested and reduction in measurement period for detecting the defect has still been requested, even when highly accurate process capable of effecting judgement with high accuracy is used, if such a process has a long processing time, such a process will become meaningless. Accordingly, there is need for providing algorism in which burden of calculation required for the image processing is reduced and high speed processing can be achieved.
Further, if there is local offset in refractive index distribution or in intensity distribution of the measurement illuminating laser beam along the sectional direction of the optical fiber, during the on-line measurement, due to the vibration of the fiber, such offset may be detected in an enlarged scale. Accordingly, there is need for providing algorism which is not influenced by such local offset.
According to a first aspect of the present invention, in a defect detecting method for detecting defect of an optical fiber, in which a laser beam is illuminated onto an optical fiber to be measured from a direction transverse to an optical axis of the fiber to check intensity distribution of forward scattering light permeated through within the optical fiber and scattered, and the defect of the optical fiber to be measured is detected on the basis of a pattern of the intensity distribution, the method is characterized in that the pattern of the intensity distribution is subjected to a smoothing process having weak smoothness and a smoothing process having strong smoothness to form first and second patterns, respectively, and a judging pattern is formed on the basis of a difference between the first and second patterns, and the defect of the optical fiber to be measured is detected by evaluating magnitude of the judging pattern.
According to a second aspect of the present invention, in a defect detecting method for detecting defect of an optical fiber, in which a laser beam is illuminated onto an optical fiber to be measured from a direction transverse to an optical axis of the fiber to check intensity distribution of forward scattering light permeated through within the optical fiber and scattered, and the defect of the optical fiber to be measured is detected on the basis of a pattern of the intensity distribution, the method is characterized in that the pattern of the intensity distribution is subjected to a smoothing process having weak smoothness and a smoothing process having strong smoothness to form first and second patterns, respectively, and a judging pattern is formed by dividing the second pattern by the first pattern, and the defect of the optical fiber to be measured is detected by evaluating magnitude of the judging pattern.
According to a third aspect of the present invention, in a defect detecting method for detecting defect of an optical fiber, a change threshold pattern associated with the first and second patterns is formed, and the magnitude of the judging pattern is evaluated by comparing the threshold pattern with the judging pattern.
According to a fourth aspect of the present invention, in a defect detecting method for detecting defect of an optical fiber, a shift averaging method is used in the smoothing process.
According to a fifth aspect of the present invention, in a defect detecting method for detecting defect of an optical fiber, in which a laser beam is illuminated onto an optical fiber to be measured from a direction transverse to an optical axis of the fiber to check intensity distribution of forward scattering light permeated through within the optical fiber and scattered, and the defect of the optical fiber to be measured is detected on the basis of a pattern of the intensity distribution, the method is characterized in that the pattern of the intensity distribution is subjected to Fourier transform to form a Fourier transform pattern, and the Fourier transform pattern is evaluated by using double logarithmic axis, and a judging curve having high contributing rate to the Fourier transform pattern is applied to the Fourier transform pattern to seek dispersion of the Fourier transform pattern from the judging curve, and the defect of the optical fiber to be measured is detected on the basis of magnitude of the dispersion.
According to a sixth aspect of the present invention, in a defect detecting method for detecting defect of an optical fiber, the pattern of the intensity distribution is subjected to a smoothing process having weak smoothness and a smoothing process having strong smoothness to form first and second patterns, respectively, and an area where a difference between the first and second patterns becomes maximum or a value obtained by dividing the second pattern by the first pattern becomes maximum is set in the pattern, and the defect of the optical fiber to be measured is detected by partially applying Fourier transform to the area in the pattern.